The name MI8 was temporarily applied to a cryptography effort mounted within the US Army during World War I. Herbert Yardley was assigned to this unit during the War, and after it continued his cryptographic work during the 1920s at what Yardley called the 'American Black Chamber' in his book of that name. It was jointly funded by the US Army and the State Department. There are occasional references to this operation as MI-8. When Secretary of State Stimson removed State Department funding (which he later explained in his memoirs, famously, as "gentlemen do not read each other's mail"), the operation was closed down.

The group name MI8, or Military Intelligence, section 8, was reused for the British signals intelligence group in World War II. Also known as the Radio Security Service, it tracked radio broadcasts about German bombers during The Blitz.

Signals intelligence in the UK is now performed by the Government Communications Headquarters, often known as GCHQ but never as MI8.